1. Field of the Invention
The present invention relates to a secondary battery, and, more particularly, to a jelly-roll type electrode assembly having an improved structure to minimize deformation occurring during winding of a negative electrode, a positive electrode, and a separator, and a secondary battery using the electrode assembly.
2. Description of the Related Art
In general, unlike primary batteries, which are not rechargeable, secondary batteries are capable of recharging. Secondary batteries are widely used in various applications, including advanced portable electronic devices such as cellular phones, notebook computers, and camcorders. Specifically, lithium secondary batteries, which operate at 3.6 V, are being actively developed. This is because the operating voltage of the lithium secondary battery is approximately 3 times higher than that of nickel-cadmium (Ni—Cd) batteries or nickel-hydride (Ni-MH) batteries, which are also widely used as power sources for electronic devices, and because they have excellent energy density per unit weight.
In general, a secondary battery employs a lithium oxide as a positive active material, and a carbon material as a negative active material. Such a secondary battery can be classified into a liquid electrolyte cell and a polymer electrolyte cell, according to the kind of electrolyte used. Batteries using a liquid electrolyte are generally referred to as lithium-ion batteries, and batteries using a polymer electrolyte are referred to as lithium-polymer batteries. The lithium secondary batteries are manufactured in various shapes, typically cylindrical, rectangular, and pouch shapes. In particular, a pouch type secondary battery, which has a pouch as a casing, is preferred because the energy density per weight and energy density per volume can be increased. Also thin, lightweight batteries can be attained, and the material cost for the casing can be reduced.
Such a conventional secondary battery having a pouch as a casing is shown in FIG. 1. Referring thereto, a secondary battery 10 includes a pouch-shaped casing 11 with an electrode assembly 20 accommodated therein. The pouch-shaped casing 11 is hermetically sealed around the electrode assembly 20.
The casing 11 includes a cover 12 and a case 13. The electrode assembly 20 is accommodated in the case 13, and the cover 12 and the case 13, which will be hermetically sealed together, are coupled to each other.
The electrode assembly 20 is configured such that a negative electrode plate and a positive electrode plate are formed by coating an active material on a current collector, drying the material, roll-pressing, and cutting. A separator is interposed between the negative and positive electrode plates, and the resultant laminate is wound in a jelly-roll type structure. A negative electrode tab 23 and a positive electrode tab 24 are led from the negative electrode plate and positive electrode plate, respectively.
In the electrode assembly having the configuration described above, winding start portions of the negative and positive electrode plates are shown in FIG. 2. Referring thereto, the negative electrode plate 21 includes a strip-shaped negative current collector 21a, a negative electrode coated portion 21b on which a negative electrode active material is formed, and a negative electrode uncoated portion 21c on which the negative electrode active material is not formed. The negative electrode coated portion and the negative electrode uncoated portion are positioned on the strip-shaped negative electrode current collector 21a. Likewise, the positive electrode plate 22 includes a strip-shaped positive current collector 22a, a positive electrode coated portion 22b on which a positive electrode active material is formed, and a positive electrode uncoated portion 22c on which the positive electrode active material is not formed. The positive electrode coated portion and the positive electrode uncoated portion are positioned on the strip-shaped positive electrode current collector 22a. The negative electrode tab 23 and the positive electrode tab 24 are fixedly disposed on the negative electrode uncoated portion 21c and the positive electrode uncoated portion 22c, respectively, by welding, and protrude from the negative electrode plate 21 and the positive electrode plate 22, respectively. The negative electrode uncoated portion 21c having the negative electrode tab 23, and the positive electrode uncoated portion 22c having the positive electrode tab 24, correspond to the winding start portions in a winding.
The thus-formed positive and negative electrode plates are wound with porous separators in a jelly roll configuration as shown in FIG. 3. Referring thereto, two sheets of separators, that is, first and second separators 25a and 25b, are first wound around a non-circular mandrel 30 by a predetermined length. The negative electrode plate 21, having the negative electrode uncoated portion 21c at its leading edge, is disposed between the first and second separators 25a and 25b which have been partially wound, and the positive electrode plate 22, having the positive electrode uncoated portion 22c at its leading edge, is disposed on the outer face of the first separator 25a. The positive electrode plate 22 is wound with the first and second separators 25a and 25b a predetermined length after the leading edge of the negative electrode plate 21.
The negative electrode uncoated portion 21c is relatively long, and is generally thinner than the positive electrode uncoated portion 22c. Thus, the negative electrode uncoated portion 21c is susceptible to deformation, e.g., crumpling, during an initial winding stage. Winding of the negative electrode uncoated portion 21c in a deformed state may cause damages to the first and second separators 25a and 25b, which face both surfaces of the negative electrode uncoated portion 21c, resulting in a short-circuit between the negative electrode plate 21 and the positive electrode plate 22, or misplacement of the negative electrode tab 23, which is attached to the negative electrode uncoated portion 21c. 